This invention relates to the field of axial solenoids and more particularly to the print wire solenoids for use in dot matrix impact type printers and the like, such as illustrated in the United States patents of Zenner et al, U.S. Pat. No. 3,729,079 or Howard, U.S. Pat. No. 3,882,986, as examples only.
A typical print head for a dot matrix type of printer may have either seven or nine wires, each operated by an individual print wire solenoid. High speed operation of such printers may require the ability to produce in excess of 600 characters per second with an average of six dots per character. An individual print wire may be required to produce in excess of 1,000 impacts per second, while maintaining a clear and distinct impact pattern.
Each impact dot produced by the wire represents a complete cycle of operation for the print wire solenoid, in which a coil is energized to move an armature from a rest position to a forward or actuated position. The print wire is carried on or operated by the armature and moved into impact with the printing medium. When the energization on the solenoid coil is removed the armature returns to its rest position. Total movement of the armature usually does not exceed 0.040" and more commonly has a movement in the range of 0.020". The return momentum of the armature must be absorbed with minimum rebound so that the unit is capable of high speed operation.
In the mass production of such solenoids it is important that they be designed so as to be produced at low cost and yet provide repeatability of design performance from unit to unit. In other words, it is important to provide a design in which the speed of operation and force of application will remain within desired limits throughout a production run. One critical design parameter of a solenoid of this type is that of providing a precise air gap between the armature and stator. Thus, it is important that the working air gap, across which the motive force is generated, be accurately maintained from unit to unit. In the past, threaded external adjustments have been provided through which a desired air gap could be reestablished after the solenoid had been assembled. The problem in maintaining a precise internal air gap has resulted from the difficulty in controlling the stack-up of the tolerances of the many assembled parts, the total axial variations which result in a loss of control of the desired air gap dimension within the assembled part.
A further difficulty which has been experienced is that of proper coupling of the print wire to the solenoid. Epoxy glues, swaging, brazing and welding have been employed with varying degrees of success and reliability. The failure of the attachment of a printing wire to the armature has been a major cause of failure of print wire solenoids.
A further difficulty which has been encountered in the print wire solenoids is that their operation may become erratic at the higher speed levels. This erratic operation can be caused by a number of factors, including friction, rebound of the armature, failure to maintain the desired air gap, and slipping of the print wire at the armature, among others.